I think it’s a good idea. If not this there should at least be an option to add or remove units from the Select all Land Military Units and Select all Visible Land Military Units hotkeys, so each player gets their own use out of it.
It’s unfair that some players can use Select All if they’re using 1 unit type (or certain units/strategies it favors like Goth Spam for example) and others can’t, purely because they’re using different units or a mix of units.
With Trebuchets, if you don’t use them you get access to Attack Move and can have all your units fighting at once and reinforcements won’t be cut off because you can spam Select-all Attack-Move constantly.
See my topic about Attack Move: Please add Attack Move and Patrol commands when Trebuchets are selected with other military units